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334                               CHEMICAL ENGINEERING
and G onto the deck along with grains of the lighter kind such as M} L, K, J and /, avoiding the overlap.
For separations below 80-mesh size and for slime separation and particularly for pilot-table work following flotation the Deister slimer table shown in Fig. 8 is very satisfactory. Owing to its extremely powerful differential motion it requires very rigid foundations, concrete being the preferable material for this purpose.
Canvas tables stationary and revolving and ordinary revolving round tables and buddies employ the third principle of concentration listed at the beginning of this section. In modern metal-milling practice these devices are falling into disuse with sulphide ores owing to the advance in flotation practice. The principle effects a separation by a film current flowing down an inclined plane or curved surface. Since the upper layers of such a film are more actively moving than
*	e
.. -'.'/-\	0
<• \. /. • • • \	0
• \x. ..:;•%	0
i^	0 0 0 Quartz
B    L   A   K        J"'         \	
FIG. 6.—Diagrammatic view, discharge corner of         FIG. 7.—Hindered settling of Wilfley table.                                             unsized material.
the lower large grains are more readily affected by it and transported by it than small. The greater also the specific gravity of the grain the less other things being equal it will be affected by the current. A small grain of high specific gravity will cling best to the separating surface. If classified feed receives treatment on this sort of device it would be expected since the gangue grains are larger that these would be carried first to waste, the grains of greater specific gravity moving more slowly or standing fast. On canvas stationary tables, the slime feed pours down the separating surface for a certain period of time after which it is diverted onto an unused deck and the remaining sand which contaminates the concentrate is washed down with a limited amount of wash water. Following this wash the concentrate is removed by washing down with strong jets. For very slimy substances there should be about 2 Ib. of water to the square foot of cleaning surface and a slope of 1K to 1 l/i in. to the foot. For coarser material less slope should be employed and 10 to 15 Ib. of water.
Magnetic Separation.—Magnetic separation dates back to 1847 or earlier and up to the advent of the Wetherill machine, the use to which the method was